The Evolution Theory, Vol. 2 of 2

by August Weismann

It has been objected to this that the guinea-pig's epilepsy may be produced by blows on the skull, and also by a destructive compression of the nervus ischiadicus through the skin, and that in both cases the epilepsy may reappear in the following generation; and this, it is supposed, shows that the intrusion of microbes is excluded. If this were so beyond a doubt, and if we could exclude the possibility that there were previously various microbes within the body, which could only penetrate into the nervous substance after the cutting or destruction of the neurilemma, nothing would be gained that would in any way support the Lamarckian principle. One could only say: Certain injuries to the nervous system give rise secondarily in guinea-pigs to morbid phenomena like epilepsy, and all sorts of functional disturbances of the nervous system often appear in the next generation, including in rare cases even the phenomena of epileptic convulsions. That this is a case of the transmission of an acquired anatomical modification brought about by the injury is not only unproved, but is decidedly negatived, for the injuries themselves are never transmitted. Thus what is transmitted must be quite different from what was acquired, for no one has ever detected in the offspring the lesion of the nerve-trunk which was cut through in the parent, or any other result except the disease to which the original injury gives rise. Moreover, the inheritance of these morbid phenomena has been again brought into dispute quite recently owing to the investigations of such experts in nervous diseases as Sommer and Binswanger, and the correctness of Brown-Séquard's results, which have dragged through the literature of the subject for so long, has been emphatically denied[11].

[11] See H. E. Ziegler's report in Zool. Centralblatt, 1900, Nos. 12 and 13.

Clearly formulated problems, like that of the inheritance of acquired characters, should not be confused by bringing into them phenomena whose causes are quite unknown. What do we know of the real causes of those central brain-irritations which give rise to the phenomena of epilepsy? It is certain enough that there are diseases which are acquired and are yet 'inherited,' but that has nothing to do with the Lamarckian principle, because it is a question of infection of the germ, not of a definite variation in the constitution of the germ. We know this with certainty in regard to the so-called Pebrine, the silkworm disease which wrought such devastation in its time; the germs of the pebrine organism have been demonstrated in the egg of the silk-moth; they multiply, not at once but later, in the young caterpillar, and it is the half-grown caterpillar, or even the moth, that succumbs to the disease.

Whether in this case also the disease germs are transmitted through the male sex-cells is not proved, as far as I am aware, but that this can happen is shown by the transmission of syphilis from father to child. That in this case, also, the exciting cause of the disease is a micro-organism cannot be doubted, although it has not yet been proved. Thus even the minute spermatozoon of Man can contain microbes, and transmit them to the germ of a new individual.

This discussion of scientific questions ought not to be brought down to the level of a play upon words, by bringing forward cases like the above as evidence for the inheritance of 'acquired characters,' as was done, for instance, by M. Nussbaum, who cited as a proof of this the migration of the alga-cells which live in the endoderm of the green freshwater Hydra into the ovum, which is originally colourless, and originates in the ectoderm of the animal ([Fig. 35B, p. 169, vol. i]). It seems to me better to make a precise distinction between the transmission of extraneous micro-organisms through the germ-cells and the handing on of the germ-plasm with the characters inherent in its structure. Only the latter is inheritance in the strict scientific sense, the former is infection of the germ.

Still less than the cases of inherited traumatic epilepsy can the morbid constitution of the children of drunkards be regarded as a proof of the inheritance of somatogenic characters, though this has often been maintained. I will not lay any stress on the fact that the allegation itself is, according to the most competent observers, such as Dr. Thomas Morton[12], far from being established. But even if it were quite certain that the numerous diseases of the nervous system, amounting sometimes to mania, which are frequently observed in the children of drunkards, were really caused by the drinking of the parents, it ought not to be overlooked that we have here to do not with the hereditary transmission of somatic variations, but of variations directly induced in the germ-plasm of the reproductive cells, for these are exposed to the influence of the alcohol circulating in the blood, just as any other part of the body is. That by this means variations in the germ-plasm can be brought about, and that these may lead to morbid conditions in the children cannot be denied, and ought not on a priori grounds to be called in question. For we are acquainted with many other influences—climatic, for instance—which directly affect and cause variation in the germ-plasm. Whether this is so in the case of drunkenness, and in what manner it comes about, whether through direct action of the alcohol, or through infection of the germ with some microbe, we must leave to the future to decide; the whole question is out of place here; it can in no way help us to clear up the problem with which we are now occupied.

[12] Morton, 'The Problem of Heredity in Reference to Inebriety,' Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894.

But even if there were not a trace of proof of the transmissibility of functional modifications, that alone would not justify us in concluding that the transmission is impossible, for many things may happen that we are not in a position to prove at present. If it could be shown that there was a whole group of phenomena that could not be explained in any other way than on the hypothesis of such inheritance, then we should be obliged to assume that it really occurred, although it was not demonstrable, and, indeed, not even theoretically conceivable. This is the standpoint of the adherents of the Lamarckian principle at present.

They say there are a great number of transformations which are simply and easily explained, if we regard them as the effects of inherited use or disuse, but which admit only of a strained explanation, and sometimes of none at all, on the basis of natural selection, and these are not a few isolated cases, but whole categories of them.

I will submit a few of these, and show at the same time why I cannot regard them as convincing, even if it be the case that we are not at present in a position to explain them without the aid of the Lamarckian principle. But let me hasten to add that it is my belief that we can do this, although certainly not without first giving a somewhat extended application to the principle of selection.